1. Field of the Invention
The present invention relates to a boundary acoustic wave filter device used as, for example, a band-pass filter of a cellular phone, and, more particularly, to a longitudinally coupled resonator boundary acoustic wave filter device.
2. Description of the Related Art
Various surface acoustic wave filter devices are used as a band-pass filter in a cellular phone or other devices.
For example, Japanese Unexamined Patent Application Publication No. 5-335881 discloses a longitudinally coupled resonator surface acoustic wave filter device used for an application of this type. FIG. 10 is a schematic plan view showing a surface acoustic wave filter device described in Japanese Unexamined Patent Application Publication No. 5-335881. The surface acoustic wave filter device 1001 includes a piezoelectric substrate 1002 made of 36 degrees rotated Y-cut X-propagating LiTaO3.
First and second longitudinally coupled resonator surface acoustic wave filter portions 1003 and 1004 are provided on the piezoelectric substrate 1002.
The first longitudinally coupled resonator surface acoustic wave filter portion 1003 includes a first interdigital transducer (IDT) 1020, and second and third IDTs 1030 and 1040. The first IDT 1020 is arranged between the second and third IDTs 1030 and 1040. The second and third IDTs 1030 and 1040 are arranged respectively on both sides of the first IDT 1020 in a surface acoustic wave propagating direction in which a surface acoustic wave propagates. Reflectors 1050 and 1060 are respectively arranged on both sides of the region in which the IDTs 1020 to 1040 are provided in the surface acoustic wave propagating direction. A first end of the middle first IDT 1020 is connected to an input terminal 1005, and a second end thereof is connected to a ground.
First ends of the IDTs 1030 and 1040 are connected to a ground, and second ends thereof are connected to one end of second and third IDTs 1031 and 1041 of the second longitudinally coupled resonator surface acoustic wave filter portion 1004.
The second ends of the IDTs 1031 and 1041 are connected to a ground.
The second longitudinally coupled resonator surface acoustic wave filter portion 1004 is also a three-IDT type longitudinally coupled resonator surface acoustic wave filter portion, and includes a first IDT 1021 between the second and third IDTs 1031 and 1041. A first end of the IDT 1021 is connected to a ground, and a second end thereof is connected to an output terminal 1006. Reflectors 1051 and 1061 are respectively arranged on both sides of the portion in which the IDTs 1021 to 1041 are provided.
In the longitudinally coupled resonator surface acoustic wave filter device 1001, where the overlap width of the electrode fingers of each of the IDTs 1020 to 1040 and 1021 to 1041 is W, and the pitch of the electrode fingers is P, the ratio W/P is in the range from 15 to 90. That is, FIG. 11 shows that, as the ratio W/P increases, an insertion loss tends to increase irrespective of a center frequency. In addition, when the ratio W/P is less than 15, an insertion loss abruptly increases irrespective of a center frequency. Japanese Unexamined Patent Application Publication No. 5-335881 describes that an insertion loss may be reduced by setting the lower limit of the ratio W/P to 15 and the upper limit of the ratio W/P to 90. Then, by setting W/P in the range of 15 to 90, insertion losses in the center frequencies may be less than or equal to 3 dB.
On the other hand, WO98/52279 describes a one-port boundary acoustic wave resonator that is arranged so that an electrode having IDTs is provided at a boundary between a piezoelectric substrate made of 36 degrees rotated Y-cut X-propagating LiTaO3 and an SiO2 film. In the description of FIG. 4 of WO98/52279, a configuration in which the aperture length of the IDT is 15λ and the interval λ of the electrode fingers of the IDT is 4 μm in the one-port boundary acoustic wave resonator is described. Note that the aperture length corresponds to a value obtained by adding the overlap width of electrode fingers of the IDT and the sum of the gap lengths between the overlap portion and the bus bars.
As described above, Japanese Unexamined Patent Application Publication No. 5-335881 describes that in a longitudinally coupled resonator surface acoustic wave filter device that utilizes a 36 degrees rotated Y-cut X-propagating LiTaO3 substrate, when the ratio W/P ranges from 15 to 90, an insertion loss may be reduced.
On the other hand, in a band-pass filter used in an RF stage of a cellular phone, a low loss is highly required. For example, when used as an RX band-pass filter in the GSM 800 MHz band or the GSM 1900 MHz band, an insertion loss of the pass band must be 2.5 dB or less. Thus, a filter device needs to be designed to satisfy such low-loss requirement. However, design parameters and values that achieve such a low loss are different between a surface acoustic wave filter device and a boundary acoustic wave filter device. Thus, even when the results of the surface acoustic wave filter device described in Japanese Unexamined Patent Application Publication No. 5-335881 are applied directly to the boundary acoustic wave filter device, an insertion loss may not be 2.5 dB or less.
On the other hand, WO98/52279 describes that in a boundary acoustic wave filter device that utilizes a 36 degrees rotated Y-cut X-propagating LiTaO3 substrate, when the aperture length is set to 15λ, a favorable characteristic may be obtained.
The inventors of the present invention studied the disclosure of the above-described related references and discovered that in a longitudinally coupled resonator boundary acoustic wave filter device that utilizes a piezoelectric substrate made of LiNbO3 having a principal plane obtained by rotating the Y-axis through 15 degrees +−10 degrees, the aperture length is set to 15λ. However, lateral leakage increases and a reduced insertion loss cannot be achieved.